Question 629012
<pre>
g = f² - 40²

factor the right side:

g = (f - 40)(f + 40)

f has to be older that 40; otherwise, (f - 40)(f + 40) would not be positive.

If f were 42 or older, the grandmother would be 164 or older, which nobody
can be.  So the father can only be 41

g = (41 - 40)(41 + 40)

g = (1)(81)

g = 81

And that is a perfect square.  So the grandmother is 81.

Edwin</pre>